1. Field of the Invention
This invention relates to a stacked tube type heat exchanger comprising flat tubes formed by extrusion molding.
2. Prior Art
Generally, conventional stacked tube type heat exchanger comprises a plurality of flat tubes and fins which are alternately stacked in parallel to one another, and ends of each stacked tube are inserted respectively into tube insertion holes provided in header tanks to be assembled, and the fins are joined together with the flat tubes, and the flat tubes are connected with the header tanks, by brazing. Then, a heat exchange medium flows between an inlet joint and an outlet joint of the header tanks in a serpentine form by making a plurality of turns.
With such conventional stacked tube type heat exchanger, flat tubes are made of aluminum material and aluminum alloy material (hereinafter called "aluminum alloy"), and a plurality of panel walls are provided inside each flat tube in a direction of the width of the inner flow passage to define a plurality of paneled flow passages. Such flat tube is formed by extrusion molding.
Generally, such a flat tube is made of, for example, aluminum alloy of JIS (Japanese Industrial Standards) No. A1050, and the fin is made of another aluminum alloy. In order to improve corrosion resistance of the flat tube, a layer of Zn is provided on the tube surface by thermal spraying of Zn over the outer surface of the extrusion molded tube. Recently, brazing of an assembled heat exchanger is often made by using non-corrosive flux of fluorides in a non-oxidizing atmosphere (for example, Nokolok Method).
However, for such stacked tube type heat exchanger which is adapted to be loaded on a vehicle, demands for lowering the cost and making it light in weight are increasing. But, with the above-described conventional method the Zn layer is provided over the tube surface by thermal spraying of Zn thereon which is an extra process employed after forming the flat tube by extrusion molding. Thus, the conventional method requires such extra process, which, in turn, requires an additional cost. Consequently, an improvement has been sought. Generally, the flat tube made of aluminum alloy of JIS No. A1050 has dimensions of 0.35 mm wall thickness, 1.3 mm space between inner walls and 2.0 mm tube thickness, and pitting corrosion occurs if the Zn thermal spraying is not applied, and such pitting corrosion comes through the tube surface.
The object of the present invention is, therefore, to provide a stacked tube type heat exchanger which assures the tube's resistance to pitting corrosion, even though the flat tube surface has no Zn layer, and accordingly, the conventional extra process of thermal spraying Zn over the outer surface of the tube during manufacture is eliminated.